This invention relates generally to turbines and more particularly, to a method and apparatus for tuning a natural frequency response of turbine blading.
A rotating turbine blade, also known as a bucket, converts energy from flowing fluid into mechanical energy. The reliability and performance of these blades is important for the successful operation of a turbine. Metallurgical examinations of failed blades show that many failures may be attributed to a fatigue of metal.
Fatigue failure may be caused by fluctuating forces in combination with steady forces. More specifically, turbine blades may experience fluctuating forces when they rotate through non-uniform fluid flow downstream from stationary vanes, also known as nozzles, positioned between adjacent rows of blades. A basic design consideration for turbines is to avoid or to facilitate minimizing resonance with natural frequencies, and the dynamic stresses produced by fluctuating forces.
The dynamic behavior of a rotating turbine blade, row of blades or the bladed disc assembly, to which the blades are coupled, may be predicted using vibration analysis of mechanical structures. In some known methods of blade design, a natural frequency analysis is based on an assumption of a single beam cantilevered at the blade root. In some other known methods, groups of blades are connected by shrouding. However, because the groups behave as a system, many more natural frequencies and modes exist which may not be predicted using single blade analysis. Moreover, the magnitude of frequencies and the number of modes depends on the number of blades in the group and the stiffness of the shrouding.
Each blade on a rotating turbine disc experiences a dynamic force when rotated through a non-uniform flow from stationary vanes. As blades rotate through areas of non-uniform flow, they may exhibit a dynamic response, such as, for example, stress, displacements, etc.
Additionally, a turbine bladed disc may be induced into a state of vibration wherein the energy build up is a maximum. This is exemplified by areas of the blade or disc where the stress or displacement is at a maximum level, and the resistance to the exciting force of the blade or disc is at a minimum. Such a condition is known as a state of resonance. When analysis or empirical testing indicates a turbine rotor may encounter a resonance condition during operation of the turbine, steps may be taken to facilitate minimizing the probability of encountering resonance. Some known methods of altering a resonance response in a rotor include changing the number of blades in a packet, changing the number of nozzles, modifying blade flexibility, modifying wheel flexibility, changing shroud flexibility, and move the operating speed range. However, such methods may only be cost-effective during a design phase of the turbine and are impractical after the design of the turbine components has been fixed.
In one aspect, a method of modifying a rotor blade for a steam turbine to facilitate altering a natural vibratory frequency of the rotor blade is provided. The rotor blade includes a leading edge, a trailing edge, a first sidewall, and a second sidewall, wherein the first and second sidewalls are connected axially at the leading and trailing edges, and extend radially between a rotor blade root and a rotor blade tip. The method includes determining a vibratory resonance condition of the rotor blade, and forming a blade extension between the rotor blade root and the rotor blade tip that alters the determined resonance condition.
In another aspect, a rotor blade for a steam turbine that includes at least one stage is provided. More specifically, the stage includes a row of rotor blades and a row of adjacent stationary nozzles, each said rotor blade includes a leading edge, a trailing edge, a first sidewall, and a second sidewall wherein, the first and second sidewalls are connected axially at the leading and trailing edges, and extend radially between a rotor blade root and a rotor blade tip wherein the rotor blade includes a first natural frequency, and a blade extension that modifies a natural frequency of the rotor blade from the first natural frequency to a second natural frequency different from the first natural frequency wherein the extension protrudes from at least one of the leading edge, the trailing edge, the first sidewall, and the second sidewall.
In yet another aspect, a multi-stage steam turbine is provided. The turbine includes a plurality of rows of rotor blades coupled circumferentially around a turbine rotor shaft. Each blade includes a leading edge, a trailing edge, a first sidewall, and a second sidewall, the first and second sidewalls are connected axially at the leading and trailing edges, and extend radially between a rotor blade root and a rotor blade tip, adjacent rows of rotor blades are separated by a row of stationary nozzles that extend circumferentially between adjacent rows of rotor blades, and each rotor blade includes a blade extension that modifies a natural frequency of said rotor blade from a first natural frequency to a second natural frequency different from said first natural frequency, said extension protruding from at least one of said leading edge, said trailing edge, said first sidewall, and said second sidewall.